feat: add /insights command with usage analytics and cost estimation

Inspired by Claude Code's /insights, adapted for Hermes Agent's multi-platform
architecture. Analyzes session history from state.db to produce comprehensive
usage insights.

Features:
- Overview stats: sessions, messages, tokens, estimated cost, active time
- Model breakdown: per-model sessions, tokens, and cost estimation
- Platform breakdown: CLI vs Telegram vs Discord etc. (unique to Hermes)
- Tool usage ranking: most-used tools with percentages
- Activity patterns: day-of-week chart, peak hours, streaks
- Notable sessions: longest, most messages, most tokens, most tool calls
- Cost estimation: real pricing data for 25+ models (OpenAI, Anthropic,
  DeepSeek, Google, Meta) with fuzzy model name matching
- Configurable time window: --days flag (default 30)
- Source filtering: --source flag to filter by platform

Three entry points:
- /insights slash command in CLI (supports --days and --source flags)
- /insights slash command in gateway (compact markdown format)
- hermes insights CLI subcommand (standalone)

Includes 56 tests covering pricing helpers, format helpers, empty DB,
populated DB with multi-platform data, filtering, formatting, and edge cases.

cli.py

@@ -1858,6 +1858,8 @@ class HermesCLI:
             self._manual_compress()
         elif cmd_lower == "/usage":
             self._show_usage()
+        elif cmd_lower.startswith("/insights"):
+            self._show_insights(cmd_original)
         elif cmd_lower == "/paste":
             self._handle_paste_command()
         elif cmd_lower == "/reload-mcp":
@@ -1983,6 +1985,39 @@ class HermesCLI:
             for quiet_logger in ('tools', 'minisweagent', 'run_agent', 'trajectory_compressor', 'cron', 'hermes_cli'):
                 logging.getLogger(quiet_logger).setLevel(logging.ERROR)
 
+    def _show_insights(self, command: str = "/insights"):
+        """Show usage insights and analytics from session history."""
+        # Parse optional --days flag
+        parts = command.split()
+        days = 30
+        source = None
+        i = 1
+        while i < len(parts):
+            if parts[i] == "--days" and i + 1 < len(parts):
+                try:
+                    days = int(parts[i + 1])
+                except ValueError:
+                    print(f"  Invalid --days value: {parts[i + 1]}")
+                    return
+                i += 2
+            elif parts[i] == "--source" and i + 1 < len(parts):
+                source = parts[i + 1]
+                i += 2
+            else:
+                i += 1
+
+        try:
+            from hermes_state import SessionDB
+            from agent.insights import InsightsEngine
+
+            db = SessionDB()
+            engine = InsightsEngine(db)
+            report = engine.generate(days=days, source=source)
+            print(engine.format_terminal(report))
+            db.close()
+        except Exception as e:
+            print(f"  Error generating insights: {e}")
+
     def _reload_mcp(self):
         """Reload MCP servers: disconnect all, re-read config.yaml, reconnect.